I'm actually using Adam Pash' mouser for a few years now on vimkeys with a small patch to improve the performance.
A few days ago me and Marijn Koesen talked about an OSX / platform independent version.
We used to talk about this before, but now being familiar with wxWidgets I realised creating a clone in C++ was probably easy.

My first windows-only version used a simple timer and a few tricks to prevent keypresses from escaping to the active window. The current version is now improved, and no longer uses a timer. I was also able to minimize an artifact the original mouser also had with the flickering of the blue shapes; it is really fast and transparency appears not to slow it down either (it did on my PC with the original mouser).

The only thing missing is multiple-monitor support at this moment, but give us a few days. Having only one monitor isn't helping .

EDIT 11-04-2010: A nice SuperMouser milestone day: Marijn added undo key 'U'. I added multiple monitor support for windows, 'M' key. The latter in need of some testing (I tested it by hardcode-dividing my screen into two halfs).

Marijn Koesen is collaborating and also adding OSX support. The GTK2 version is coming soon. And multiple-monitor support currently has highest priority.

It's all on github for download.
The binary for windows is on github as well.